Anal. Chem. 1997, 69, 3128-3129
Correspondence
Dimethylcarbazoles in Crude Oils: Comment on “Liquid Chromatographic Separation Schemes for Pyrrole and Pyridine Nitrogen Aromatic Heterocycle Fractions from Crude Oils Suitable for Rapid Characterization of Geochemical Samples” SIR: In an evaluation of chromatographic methods for the isolation of pyrrolic and pyridinic nitrogen compounds from crude oils, Li et al.1 reported peak assignments for carbazole, the methylcarbazoles, and several C2-alkylcarbazoles. During standard compound coinjection studies using gas chromatography/mass spectrometry, we became aware that the peak assignments reported by Li et al.1 for several of the C2alkylcarbazoles are incorrect. The peak assignments made by these authors for carbazole, the methylcarbazoles, 1,8-, 2,4-, and 2,5-dimethylcarbazole, 1-ethylcarbazole, benzo[a]carbazole, and benzo[c]carbazole, however, have been confirmed. Figure 1 shows the m/z 195 mass chromatogram (molecular ion for C2-alkylcarbazoles) of the nitrogen compound fraction isolated from a production oil from the Miller oilfield (North Sea) using the solid-phase extraction procedure described by Larter et al.2 The peak assignments for the majority of the compounds indicated in Figure 1 were made on the basis of coinjections with recently synthesised standard compounds authenticated by NMR analysis. The standards were also assayed for purity using capillary gas chromatographic analysis. The elution sequence for the C2-alkylcarbazoles is in close agreement with those reported by Ignatiadis et al.3 and Dorbon et al.4 It should be noted that the assignment of peaks labeled “1,3”, “1,6”, and “1,7” in Figure 1 is based on the elution order given by these authors, and not determined through coinjection by us. Due to the unavailability of standard compounds, we are unable to assign the 3,5- and 3,6-dimethylcarbazoles as
Figure 1. Mass chromatogram (m/z 195) of nitrogen compound fraction from Miller crude oil showing peak assignments for dimethyland ethylcarbazoles. Key: 1,8 ) 1,8-dimethylcarbazole, etc.; 1-ET ) 1-ethylcarbazole, etc. Analytical conditions: VG Micromass TRIO 1000 GC/MS; fused silica capillary column, HP-5 (Hewlett Packard) 30 m × 0.25 mm i.d. × 0.25 µm film thickness; helium carrier gas at 25 kPa column inlet pressure, column flow 1 mL/min; GC oven program, 40 °C, initial hold time 3 min, then 10 °C/min to 200 °C (1 min hold), then 4 °C/min to 310 °C, final hold time 13 min; sample injection, splitless (1 min) then split (20 mL/min); injector temperature, 280 °C; GC/MS interface temperature, 310 °C; MS source temperature, 250 °C; electron impact mode, electron energy ) 70 eV; operating mode; selected ion recording; resolution, 1000, unit mass; data acquisition, Masslab (VG Labsystems).
(1) Li, M.; Larter, S. R.; Stoddart, D.; Bjorøy, M. Anal. Chem. 1992, 64, 13371344. (2) Larter, S. R.; Bowler, B. F. J.; Li, M.; Chen, M.; Brincat, D.; Bennett, B.; Noke, K.; Donohoe, P.; Simmons, D.; Kohnen, M.; Allan, J.; Telnaes, N.; Horstad, I. Nature 1996, 383, 593-597. (3) Ignatiadis, I.; Schmitter, J. M.; Arpino, P. J. Chromatogr. 1985, 324, 87111. (4) Dorbon, M.; Ignatiadis, I.; Schmitter, J.-M.; Arpino, P.; Guiochon, G.; Toulhoat, H.; Huc, A. Fuel 1984, 63, 565-570.
reported in the original publication. We are, however, able to “assign” two additional ethyl-substituted carbazoles, 3-ethylcarbazole and 4-ethylcarbazole. Under the analytical conditions used (see Figure 1, legend), 4-ethylcarbazole coeluted with 1,4-dimethylcarbazole, and 3-ethylcarbazole coeluted with 1,5-dimethylcarbazole. Coinjection studies using a longer column (50 m BPX5, SGE) suggest that 3-ethylcarbazole is present in Miller oil, eluting slightly after (but well resolved from) 1,5-dimethylcarbazole. The 4-ethylcarbazole, however, coelutes with 1,5-dimethylcarbazole, and we are, as yet, unable to establish its presence in this oil sample. The possible occurrence of these ethylcarbazoles in oils and the associated coelution problems carry cautionary implications for quantification. Li et al.1 suggested that changes in the relative abundance of individual methyl- and dimethylcarbazole isomers may be controlled by maturation and/or migration, with preferential sorptive retention of NH-exposed isomers by the source rock resulting in their depletion in a migrating oil and their enhancement in source rock extracts. Two dimethylcarbazole ratios involving NH-shielded (1,8-), NH-partially shielded (1,4-) and NH-exposed (2,6-) isomers were described in Table IV of Li et al.1 The revised peak assignments reported here indicate that the previously identified NH-partially shielded 1,4-dimethylcarbazole is, in fact, the 1,3-isomer (also partially shielded); the NH-exposed 2,6-dimethylcarbazole is, in fact, the 1,4-isomer (partially shielded); and the NH-exposed 3,5-dimethylcarbazole is, in fact, the 2,7-isomer (also NH-exposed).
3128 Analytical Chemistry, Vol. 69, No. 15, August 1, 1997
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The main geochemical conclusion of the original report, that the migrated oils studied were enriched in shielded and partially shielded isomers (e.g., 1,8-dimethylcarbazole, 1-methylcarbazole, and 1-ethylcarbazole), remains unchanged. ACKNOWLEDGMENT We gratefully acknowledge Dr. Eugene Frolov (Russian Academy of Sciences), who synthesized and authenticated the C2alkylcarbazole standards. Paul Donohoe (NRG) and Ulrich Disko (ICG-4) assisted with the GC/MS analysis; Dr. Stuart Hill (University of Newcastle) and Dr. Barry Bennett (NRG) performed NMR analysis and interpretation; and Barbara Brown and Yvonne Hall prepared the artwork and manuscript.
Bernard F. J. Bowler and Stephen R. Larter*
Newcastle Research Group in Fossil Fuels and Environmental Geochemistry (NRG), Drummond Building, The University, Newcastle upon Tyne NE1 7RU, U.K. Heather Clegg, Heinz Wilkes, and Brian Horsfield
Institut fu¨ r Chemie und Dynamik der Geospha¨ re (KFA), Forschungszentrum Ju¨ lich GmbH, ICG-4, D-52425 Ju¨ lich, Germany Maowen Li Geological Survey of Canada, 303 33rd Street NW, Calgary, Alberta T2L 2A7, Canada AC9703358
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